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Serial Communications
Appendix C- Asynchronous Communications
Serial data communications implies that individual bits of a character are transmitted consecutively to a receiver that
assembles the bits back into a character. Data rate, error checking, handshaking, and character framing (start/stop bits) are
pre-defined and must correspond at both the transmitting and receiving ends.
Asynchronous communications is the standard means of serial data communication for PC compatibles and PS/2 computers.
The original PC was equipped with a communication or COM: port that was designed around an 8250 Universal
Asynchronous Receiver Transmitter (UART). This device allows asynchronous serial data to be transferred through a simple
and straightforward programming interface. A start bit, followed by a pre-defined number of data bits (5, 6, 7, or 8) defines
character boundaries for asynchronous communications. The end of the character is defined by the transmission of a pre-
defined number of stop bits (usually 1, 1.5 or 2). An extra bit used for error detection is often appended before the stop bits.
Remain Idle or
next start bit
Odd, Even
or
Unused
STOP
P
BIT
5 to 8 Data Bits
Idle state of
line
1
0
1
1.5
2
Figure 7 - Asynchronous Communications Bit Diagram
This special bit is called the parity bit. Parity is a simple method of determining if a data bit has been lost or corrupted during
transmission. There are several methods for implementing a parity check to guard against data corruption. Common methods
are called (E)ven Parity or (O)dd Parity. Sometimes parity is not used to detect errors on the data stream. This is refereed to
as (N)o parity. Because each bit in asynchronous communications is sent consecutively, it is easy to generalize asynchronous
communications by stating that each character is wrapped (framed) by pre-defined bits to mark the beginning and end of the
serial transmission of the character. The data rate and communication parameters for asynchronous communications have to
be the same at both the transmitting and receiving ends. The communication parameters are baud rate, parity, number of data
bits per character, and stop bits (i.e. 9600,N,8,1).
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